Add default labels to MR template

What does this Merge Request do?

Assuming that every MR targeting this project should have the following labels:

• Category:GDK
• sectiondev
• devopscreate
• ~"group::ecosystem"

Then this update to the default MR template will save typing and make labelling more consistent.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.